Integrative optimization of chirped mirrors for intracavity dispersion compensation

A new integrative optimization of three chirped mirrors for precise broadband intracavity group delay dispersion compensation in ring cavity is proposed. Simulation demonstrates that the residual group delay dispersion ripples of these three chirped mirrors are less than 25 fs² for most of the wavelengths from 600 to 1200 nm. Pulse evolution simulation shows that these three matched chirped mirrors compensate the group delay dispersion in the Ti:Sapphire ring cavity laser well to obtain an octave-spanning spectrum.     

Design of multilayer high-dispersion mirrors using multi-swarm optimization method

Design of high-dispersion mirrors (HDMs) using a proposed multi-swarm optimization method is reported. We design HDMs for Yb:YAG disk oscillator at 1030 nm and ultrashort pulse Cr:YAG laser at 1550 nm. The results show that the optimum group delay dispersion and reflectance can be obtained with optimal number of layers. The proposed optimization method has a fast convergence rate and powerful global search ability and can be utilized effectively for the design of a variety of optical thin film filters.     

Large-area broadband saturable Bragg reflectors by use of oxidized AlAs

Broadband saturable Bragg reflectors (SBRs) are designed and fabricated by monolithic integration of semiconductor saturable absorbers with broadband Bragg mirrors. The wet oxidation of AlAs creates low-index AlxOy layers for broadband, high-index-contrast AlGaAs/AlxOy or InGaAlP/AlxOy mirrors. SBR mirror designs indicate greater than 99% reflectivity over bandwidths of 294, 466, and 563 nm for center wavelengths of 800, 1300, and 1550 nm, respectively. Highly strained and unstrained absorbers are stably integrated with the oxidized mirrors. Large-scale lateral oxidation techniques permit the fabrication of SBRs with diameters of 500 microm. Large-area, broadband SBRs are used to self-start and mode lock a variety of laser systems at wavelengths from 800 to 1550 nm.     

Negative dispersion mirrors for dispersion control in femtosecond lasers: chirped dielectric mirrors and multi-cavity Gires-Tournois interferometers

Chirped dielectric laser mirrors have been known for years as useful devices for broadband feedback and dispersion control in femtosecond pulse lasers. First we present a novel design technique referred to as frequency domain synthesis of chirped mirrors. These mirrors exhibit high reflectivity and nearly constant group delay over 150 THz supporting generation of sub-5-fs pulses in the visible. Afterwards, multi-cavity thin-film Gires-Tournois interferometers are introduced for the first time as an alternative approach to realize "negative dispersion mirrors". These novel dielectric high reflectors exhibit reflectivities R>99.97% and a negative group delay dispersion of ⫆ǃ fs² over a bandwidth of 56 THz. Dispersive properties originate from coupled resonances in multiple 5/2 cavities embedded in the layer structure. The general structure and performance of multi-cavity Gires-Tournois interferometers are compared to that of chirped dielectic mirrors and their distinct applications are discussed.     

Sub-two-cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser

Pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW. Low-dispersion prisms and double-chirped mirrors provide broadband controlled dispersion and high reflectivity. These pulse durations are to our knowledge the shortest ever generated directly from a laser oscillator.     

Theoretical investigation of chirped mirrors in semiconductor lasers

This paper reports a theoretical design of chirped mirrors in 1.3-μm double-section semiconductor lasers to achieve high reflectivity and dispersion compensation over a broad bandwidth. Analytic expressions for reflectivity, group delay and group delay dispersion are derived. We use for the first time chirped air/semiconductor layer pairs as mirrors for higher-order dispersion compensation in semiconductor lasers. Our optimised calculations demonstrate that the broad-band mirrors designed consist of a total of only 12 air/semiconductor layers and achieve a reflectivity higher than 99.8%, a smooth group delay and almost stable dispersion in the laser cavity over a 100-nm bandwidth. Due to a high index contrast of both types of the layers, n _l = 1, n _h~ 3.5, a high-reflectivity bandwidth of > 700 nm is obtained in 1.3-μm semiconductor lasers. We also compare our results with that of a commercial simulation program and show a good agreement between them. As a conclusion, we assume from the theoretical results that air/semiconductor layer pairs with varying thicknesses used at one end of double-section semiconductor lasers can lead to femtosecond optical pulse generation using mode-locking techniques. An erratum to this article can be found at .     

一种有效的色散补偿镜优化设计方法

介绍了一种有效的色散补偿镜优化设计方法.这种方法将广义模拟退火法和最小二乘法结合起来使用.在色散补偿镜的设计过程中,用双啁啾结构作为初始膜系,发现了一些设计双啁啾结构应注意的规律.这种新的优化方法避免了局部陷阱问题,最终结果也比较令人满意.此外,这种优化设计方法还可以应用于设计啁啾光纤光栅.     

双啁啾介质镜的设计与分析

从双啁啾膜系原理出发，经过自行编制的软件优化计算，得到了由54层介质膜组成的具有色散补偿功能的宽带高反镜，高反射率（大于99.9%）带宽宽达120nm，群延迟色散量为－50fs^2。对基于耦合模理论和传输线理论的双啁啾镜的原理和设计思路进行了分析，并通过计算机模拟验证了设计结果，分析了膜系各参量的改变对膜系的反射率和时延特性的影响。另外，在优化计算过程中发现了一些值得注意的有用的规律。     

Design of metaporous supercells by genetic algorithm for absorption optimization on a wide frequency band

The optimization of acoustic absorption by metaporous materials made of complex unit cells with 2D resonant inclusions is realized using genetic algorithm. A nearly total absorption over a wide frequency band can be obtained for thin structures, even for frequencies below the quarter wavelength resonances i.e., in a sub-wavelength regime. The high absorption performances of this material are due to the interplay of usual visco-thermal losses, local resonances and trapped modes. The density of resonant and trapped modes in this dissipative porous layer, is a key parameter for broadband absorption. The best configurations and critical coupling conditions are found by genetic algorithm optimization. Several types of resonators are included gradually in the studied configurations (split-rings, Helmholtz resonators, back cavities) with increasing complexity. The optimization leads to a metaporous structure with a 2-cm sub-wavelength layer thickness, exhibiting a nearly total absorption between 1800 Hz and 7000 Hz. The influence of the incidence angle on the absorption properties is also shown.     

Generation of sub-10-fs pulses from a Kerr-lens mode-locked Cr(3+):LiCAF laser oscillator by use of third-order dispersion-compensating double-chirped mirrors

Pulses as short as 9 fs at 220-mW average power and a 97-MHz repetition rate are generated from a cw Ti:sapphire-pumped Kerr-lens mode-locked Cr(3+)LiCAF laser oscillator employing broadband double-chirped mirrors for second- and third-order dispersion compensation. Fine adjustment of dispersion is accomplished with a fused-silica prism pair. The result demonstrates that Raman-induced self-frequency shifting of the pulse does not limit sub-10-fs pulse generation from colquiriite crystals.     

Broadband Terahertz Polarization Converter and Asymmetric Transmission Based on Coupled Dielectric‐Metal Grating

Coupled dielectric‐metal gratings are investigated for broadband terahertz (THz) wave polarization conversion and asymmetric transmission by the experiments and numerical simulations, which are composed of the subwavelength Si grating and metallic wire grating layers. The dielectric grating layer with a large artificial birefringence and low dispersion is employed as a phase engineered waveplate, and the metal wire grating arranged with a 45° angle to the dielectric grating is utilized as a high‐efficiency polarizer. Due to the subwavelength integration, this coupled grating presents a local resonance coupling mechanism between dielectric and metal gratings, which greatly enhances the polarization rotation and expands the bandwidth, not a simple combination with dielectric and metallic gratings. The results demonstrate that a broadband asymmetric transmission with an extinction ratio of 30dB from 0.2 to 1.2 THz is achieved and the highest transmission of 90% can be obtained. It provides a simple way towards practical applications for THz artificial dispersion materials, polarization control and asymmetric transmission.     

Metamaterial coatings for broadband asymmetric mirrors

We report the design and fabrication of nanostructured metal-dielectric mirrors with high reflectance asymmetries in the visible spectral range. Applying dispersion engineering principles to model a broadband and large reflectance asymmetry, we obtain a dielectric function for this metamaterial, closely resembling the effective permittivity of disordered metal-dielectric nanocomposites. Coatings realized by using disordered nanocrystalline silver films on glass substrates confirm the theoretical predictions, exhibiting symmetric transmittance, accompanied by large broadband reflectance asymmetries.     

Flat gain spectrum design of Raman fiber amplifiers based on particle swarm optimization and average power analysis technique

In this work, the modified particle swarm optimization is used as an optimization tool to determine the set of wavelengths and power levels of pumps that delivers a flat gain spectrum for Raman fiber amplifiers. The average power analysis technique is used as a numerical method to solve the coupled Raman amplifier equations. By combining the modified particle swarm optimization and average power analysis technique an efficient algorithm for the design of flat-gain-spectrum broadband Raman fiber amplifiers is constructed. Application of this algorithm to the design of flat-gain-spectrum broadband Raman fiber amplifiers shows that the design efficiency of the new method is improved by 1-2 orders of magnitude compared with similar implementations previously reported in the literature. A 4-backward-pump gain-flattened Raman amplifier with bandwidth of 100-nm and maximum gain ripple of <1.0 dB is designed to demonstrate the technique.     

融合插值点优化的多项式结构宽带波束形成器设计方法

多项式结构设计方法是主瓣指向可调宽带波束形成器设计的一类重要方法。多项式结构的阶数是有限的,导致主瓣实际指向与期望指向之间存在偏差,因而影响了波束形成器的指向性指数。针对这一问题,该文提出了一种基于插值点优化的多项式结构宽带波束形成器设计方法。首先,引入多项式结构插值点处阵列响应的空间导数约束,以减小主瓣指向偏差;进而利用粒子群优化算法对多项式结构中的插值点进行优化,以充分利用插值点位置提供的自由度进一步提升多项式结构宽带波束形成器的性能。优化设计结果表明,与现有设计方法相比,该文提出的方法不仅降低了主瓣的指向偏差,同时也提高了指向性指数,有效改善了多项式结构宽带波束形成器的性能。     

Group delay dispersion compensation in an Ytterbium-doped fiber laser using intracavity Gires-Tournois interferometers

This paper reports the use of a pair of multi-cavity Gires-Tournois interferometer (GTI) mirrors in a ring cavity Ytterbium-doped fiber laser for dispersion compensation. The optical spectrum was broadened by 6 nm due to the large negative group delay dispersion introduced by the light undergoing 10 round trips between the GTI mirrors. The design of the GTI mirrors and the optimisation of the layers are discussed and fabrication results are presented.     

Optimization of platinum-carbon multilayer mirrors for hard X-ray optics

The design of hard X-ray optics for astrophysical technology is one of the key technologies for investigating active galaxies and clusters of galaxies. In the present paper, we have optimized multilayer mirrors of platinum-carbon layer pairs for the hard X-ray region at different grazing angles. The Luus-Jaakola optimization procedure has been implemented for the global optimization of multialyer mirrors. With this algorithm it is not necessary to specify initially the number of layers present in a design.     

Compression of 2 mJ kilohertz laser pulses to 17.5 fs by pairing double-prism compressor: analysis and performance

A general expression of dispersion for pairing double-prism compressor with arbitrary apex angle is presented and a limitation to generate negative group-delay dispersion (GDD) for broadband spectrum is obtained. By use of this compressor with spectral shaping filters and chirped mirrors, 2 mJ, 17.5 fs pulses from a compact 10-pass kilohertz Ti:sapphire amplifier system are generated.     

基于拓扑优化设计的宽频吸波复合材料

本文基于拓扑优化方法设计并制备了一种宽频吸波复合材料,该吸波复合材料由高强玻璃纤维透波板、电阻损耗型超材料、聚氨酯泡沫和碳纤维反射板组成.仿真及测试结果表明,该吸波复合材料在2–18 GHz频段内的平板反射率均小于-12 dB.并且由于采用高强玻璃纤维及碳纤维复合材料作为面板层,聚氨酯泡沫作为芯材,因此该吸波复合材料不仅在较宽频带内对电磁波具有高的吸收率,同时还具有质量轻、耐高温、耐低温、耐湿热、抗腐蚀等特点,便于实现吸波与力学性能及耐环境性能的兼容,具有一定的工程应用价值.     

Time domain topology optimization of 3D nanophotonic devices

We present an efficient parallel topology optimization framework for design of large scale 3D nanophotonic devices. The code shows excellent scalability and is demonstrated for optimization of broadband frequency splitter, waveguide intersection, photonic crystal-based waveguide and nanowire-based waveguide. The obtained results are compared to simplified 2D studies and we demonstrate that 3D topology optimization may lead to significant performance improvements.     

Semiconductor saturable-absorber mirror assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime

Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.